This disclosure relates generally to equipment utilized and operations performed in conjunction with rheological testing and, in an example described below, more particularly provides a combined rheometer and mixer having helical blades.
Various geometrical configurations have been used by rheologists to characterize the flow behavior of fluids under stress in rotational viscometry. The more common ones are bob/sleeve (couette) and impeller (mixer) geometries. Complex particle laden fluids used in the oil field pose unique challenges for rheological measurement in these geometries. These fluids, frequently, are a combination of light weight materials/weighting agents, clays, elastomers, polymers, resins, salts and cementitious materials in water or oil media. These fluids exhibit a high degree of non-Newtonian behavior, are sometimes thixotropic, have particle setting/phase separation issues when not sheared uniformly and sometimes are so thick/slippery to create coring and wall slip problems in the geometries where they are investigated.
In addition to maintaining the particles in suspension, some situations may also demand that two or more fluids be “homogenized in-situ” before carrying out the rheological measurements. The coefficients to convert torque-RPM data to rheograms are known to vary with the degree of shear thinning. A wide range of literature is available to corroborate this. Process engineers determine these coefficients for the set of fluids used in the plants on rheological instrumentation to deduce/monitor process behavior under varying shear rates, well in advance of a process being conducted.
However, all oil field fluids are different and a new “recipe” is formulated and mixed every time for subterranean operations. To understand the impact of these fluids on wellbore friction pressures, their solids carrying capability, their velocity profiles and the way they interact with other neighboring fluids, it is highly recommended to: a) carry out rheological experimentation in a geometry that will accurately probe the homogenous representative sample, and b) use correct conversion coefficients to deduce rheology from torque-RPM data. Unfortunately, prior rheometer geometries lead to errors due to: a) the fluid sample being probed is not a homogenous or representative sample, and/or b) measurement errors related to wall slip, inaccurate torque measurements (e.g., a stuck spring, etc.).
Compatibility tests are performed at times to determine whether certain fluids are compatible with each other. In order to ascertain compatibility of fluids related to cementing oil and gas wells, rheological characteristics of a base fluid are measured at downhole temperature and pressure, then a predetermined quantity of a second fluid at downhole temperature and pressure is added while mixing at a predetermined volume averaged shear rate. In subterranean well operations, examples of base and second fluids could comprise drilling fluid and fluid spacer, fluid spacer and cement slurry, drilling fluid and cement slurry, etc.
It would be beneficial to be able to provide an improved rheometer capable of supplying a predefined mixing step prior to accurately measuring rheological properties of fluids. The predefined mixing step could impart an integral shear history similar to that of fluid travel in a well with known characteristics. The improved rheometer could result from adapting an existing commercial rheometer with an improved rheometer geometry. Such an improved rheometer geometry would also be useful for rheological investigation in operations other than well operations.